Several neurodegenerative diseases, collectively referred to as tauopathies, are characterized by the presence within the brain of pathological neuronal inclusions comprised of hyperphosphorylated forms of tau protein. Alzheimer?s disease (AD) is the most prevalent tauopathy with >5 million patients in the U.S., and there is compelling evidence that this tau pathology causes the neurodegeneration observed in AD and other tauopathies. There is growing interest in developing tau- directed drugs for the treatment of AD, spurred in part by the growing number of Phase 3 clinical failures of therapeutic candidates directed to the other hallmark pathology in AD, amyloid ? plaques. To date, only a small number of tau-directed drugs have progressed to clinical testing, with most of these being immunotherapeutics. A key limitation to the identification of small molecules to reduce tau pathology has been the lack of robust cell-based assays that mimic the process of tau inclusion formation and clearance. We have developed a HEK293 cell assay of tau inclusion formation, as well as a primary neuronal assay that develops AD-like tau pathology. Notably, this latter model develops abundant tau inclusions in rodent neurons through misfolding of endogenous mouse tau after seeding of the neurons with enriched pathologic tau derived from AD brain. Proof-of-concept screens of the Prestwick library of mostly approved drugs was conducted in both the HEK293 cell and neuronal assays of tau pathology. Importantly, the neuronal tau inclusion assay provided a higher number of validated hits with greater potential biological relevance than did the HEK293 cell assay. These findings confirm the importance of assessing compounds in the cell type most affected in tauopathies, and also demonstrate the value of utilizing an assay that does not require mutant tau protein overexpression. Here we propose a research program directed to the screening of a diverse set of compounds in the tau neuronal inclusion assay, with follow-up characterization in orthogonal and secondary assays that have also been developed in our laboratory. These studies will consist of an evaluation of ~7100 biologically active compounds directed to a diversity of cellular targets, with a bias toward neuronal targets. The overall objectives of this 4-year program are to 1) identify compounds directed to pathways/targets that reduce neuronal tau inclusions; 2) select compounds with favorable physico-chemical properties from 2-3 different compound target classes to test for brain exposure and pharmacokinetic properties, as well as tolerability, in mice to identify compounds that are suitable for subsequent proof-of-concept efficacy testing in a mouse model that replicates the key tau pathologies of AD; and 3) confirm through target knockdown and overexpression studies the identity of the cellular targets of compounds shown to be efficacious in the mouse model of tau pathology. These studies will provide important information about small molecules directed to targets that modulate tau pathology, and such molecules will serve as the basis for future target-based drug development programs.